The structure and ultra-structure of the cranial and spinal arachnoid villi and granulations are being studied in man and in certain animls by light microscopy, microradiography, autoradiography, scanning electron microscopy and transmission electron microscopy. These studies are being carried out after materials perfused into the subarachnoid space have passed into the absorptive pathways within the arachnoid villi and granulations. The choiroid plexus from various parts of the ventricular system is being studied by various structural and ultra-structural techniques. Physiological techniques are also being used to quantitate the rates of formation of cerebrospinal fluid in the various ventricular cavities and to study the rate of cerebrospinal fluid absorption under various conditions.